Glossary term
Hydraulics
The engineering discipline concerned with the behaviour, control, and use of liquids under pressure and flow.
Definition
conceptHydraulics is the engineering study and application of liquids at rest or in motion, especially where pressure and flow are used to transmit energy or control forces.
Hydraulics covers hydrostatics, pipe flow, open-channel flow, pumps, valves, actuators, hydraulic circuits, water infrastructure, fluid power, and measurement devices. In mechanical systems it is used to transmit large forces through pressurized oil or water. In civil and environmental systems it is used to analyse rivers, pipes, drainage networks, canals, dams, spillways, and water distribution. The same discipline links pressure, flow rate, head loss, viscosity, turbulence, cavitation, and control.
Hydraulics deals with liquids under pressure and in motion. The field includes two broad viewpoints. Hydrostatics studies fluids at rest, where pressure depends on depth, density, and gravity. Hydrodynamics studies fluids in motion, where flow rate, velocity, friction, turbulence, valves, pumps, and geometry determine behaviour.
In mechanical engineering, hydraulics often means fluid power: using pressurized liquid to transmit force and motion. A pump supplies flow, valves control direction and pressure, actuators convert hydraulic energy into motion, and reservoirs, filters, accumulators, and heat exchangers support reliability. Hydraulic systems are used in excavators, presses, aircraft actuators, elevators, machine tools, brakes, steering systems, and industrial automation.
In civil and environmental engineering, hydraulics covers open-channel flow, pipe networks, stormwater systems, dams, spillways, culverts, irrigation, sediment transport, water distribution, and flood analysis. The same basic quantities appear, but the scale, free-surface behaviour, and uncertainty can be very different.
Core relationships
Pascal’s law explains why pressure applied to a confined fluid can transmit force through a hydraulic circuit. The Bernoulli equation relates pressure head, velocity head, and elevation head under idealized conditions. Real systems require head-loss models because viscosity and turbulence dissipate energy. Reynolds number indicates whether flow is laminar, transitional, or turbulent. Valve coefficients and orifice equations relate pressure drop to flow rate.
Hydraulic design often balances force, speed, pressure drop, efficiency, heat generation, controllability, noise, and safety. Increasing flow can increase actuator speed but also raises line losses and heat. Increasing pressure can reduce actuator size but increases stress, leakage risk, seal demand, and stored energy.
Failure modes and practical issues
Common hydraulic problems include leakage, aeration, cavitation, contamination, overheating, water ingress, seal wear, valve sticking, hose failure, pressure spikes, and unstable control. Cavitation is especially damaging because local vapor bubbles collapse and erode surfaces. Contamination is a major cause of valve and pump failure; filtration and fluid cleanliness are therefore design requirements, not maintenance details.
Common mistakes
A common mistake is treating hydraulics as incompressible and instantaneous in all circumstances. Liquid compressibility, trapped air, hose expansion, valve dynamics, and long lines can create delay, oscillation, and pressure surge. Another mistake is designing for nominal flow while ignoring startup, cold oil viscosity, transient loads, emergency stops, relief valve behaviour, and heat rejection.